U.S. patent application number 10/626441 was filed with the patent office on 2004-09-23 for method and apparatus for multipath demodulation in a code division multiple access communication system.
Invention is credited to Lundby, Stein, Razoumov, Leonid, Wheatley, Charles E. III.
Application Number | 20040184513 10/626441 |
Document ID | / |
Family ID | 22668153 |
Filed Date | 2004-09-23 |
United States Patent
Application |
20040184513 |
Kind Code |
A1 |
Lundby, Stein ; et
al. |
September 23, 2004 |
Method and apparatus for multipath demodulation in a code division
multiple access communication system
Abstract
Multipath RAKE receiver structure that allows for the concurrent
demodulation of multipath signals that arrive at the receiver at
arbitrarily low arrival time differences. The fingers are set to be
a fixed offset from one another. One finger tracks the shift in the
peak of the multipath component and the additional fixed offset
fingers follow the tracking.
Inventors: |
Lundby, Stein; (San Diego,
CA) ; Razoumov, Leonid; (San Diego, CA) ;
Wheatley, Charles E. III; (Del Mar, CA) |
Correspondence
Address: |
Qualcomm Incorporated
Patents Department
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
22668153 |
Appl. No.: |
10/626441 |
Filed: |
July 23, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10626441 |
Jul 23, 2003 |
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09182367 |
Oct 27, 1998 |
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6625197 |
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Current U.S.
Class: |
375/148 ;
375/E1.032 |
Current CPC
Class: |
H04B 2201/70707
20130101; H04B 1/712 20130101; H04B 1/7117 20130101 |
Class at
Publication: |
375/148 |
International
Class: |
H04B 001/707 |
Claims
What is claimed is:
1. A spread spectrum receiver for receiving multiple spread
spectrum signals each traveling upon a different propagation path
and each having a resultant arrival time difference with respect to
one another, said spread spectrum receiver comprising: first
demodulator means for demodulating a first spread spectrum signal
of said multiple spread spectrum signals in accordance with a first
arrival time; and second demodulator means for demodulating a
second spread spectrum signal of said multiple spread spectrum
signals in accordance with a fixed time interval difference with
respect to said first arrival time.
2. The spread spectrum receiver of claim 1 wherein said first
demodulator comprises: pseudorandom noise descrambling means for
descrambling said first spread spectrum signal in accordance with a
pseudorandom noise sequence; phase adjustment means for extracting
a pilot signal from said pseudorandom noise descrambled signal and
multiplying said pseudorandom noise descrambled signal with said
pilot signal; and dechannelization means for multiplying said phase
adjusted signal by an orthogonal channel sequence.
3. The spread spectrum receiver of claim 2 further comprising Walsh
sequence generator means for generating said orthogonal channel
sequence and wherein said second dechannelization means further
comprises: delay element for receiving said orthogonal channel
sequence and for delaying said orthogonal channel sequence by said
fixed time interval to provide said second orthogonal channel
sequence.
4. The spread spectrum receiver of claim 2 wherein said phase
adjustment means comprises: pilot filter for extracting said pilot
signal from said first spread spectrum signal; and complex
conjugate multiplier means for receiving said first spread spectrum
signal and said extracted pilot signal and for multiplying said
first spread spectrum with said extracted pilot signal.
5. The spread spectrum receiver of claim 4 wherein said pilot
filter extracts said pilot signal in accordance with an orthogonal
pilot sequence.
6. The spread spectrum receiver of claim 1 further comprising a
combiner means for receiving said first demodulated spread spectrum
signal and said second demodulated spread spectrum signal and for
combining said first demodulated spread spectrum signal and said
second demodulated spread spectrum signal to provide an improved
estimate of said spread spectrum signal.
7. The spread spectrum receiver of claim 1 further comprising
switching means for providing said first spread spectrum signal to
said first demodulator means and for switching after said fixed
time interval to provide said second spread spectrum signal to said
second demodulator means.
8. A method for receiving multiple spread spectrum signals each
traveling upon a different propagation path and each having a
resultant arrival time difference with respect to one another, said
method comprising the steps of: demodulating a first spread
spectrum signal of said multiple spread spectrum signals in
accordance with a first arrival time; and demodulating a second
spread spectrum signal of said multiple spread spectrum signals in
accordance with a fixed time interval difference with respect to
said first arrival time.
9. The method of claim 8 wherein said step of demodulating said
first spread spectrum signal comprises the steps of: descrambling
said first spread spectrum signal in accordance with a pseudorandom
noise sequence; extracting a pilot signal from said pseudorandom
noise descrambled signal; multiplying said pseudorandom noise
descrambled signal with said pilot signal; and multiplying said
phase adjusted signal by an orthogonal channel sequence.
10. The method of claim 9 further comprising the steps of:
generating said orthogonal channel sequence; and delaying said
orthogonal channel sequence by said fixed time interval to provide
said second orthogonal channel sequence.
11. The method of claim 8 further comprising the step of combining
said first demodulated spread spectrum signal and said second
demodulated spread spectrum signal to provide an improved estimate
of said spread spectrum signal.
12. The method of claim 8 further comprising the steps of: first
switching to provide said first spread spectrum signal; and second
switching after said fixed time interval to provide said second
spread spectrum signal.
13. An apparatus for receiving multiple spread spectrum signals
each traveling upon a different propagation path and each having a
resultant arrival time difference with respect to one another, said
apparatus comprising: means for demodulating a first spread
spectrum signal of said multiple spread spectrum signals in
accordance with a first arrival time; and means for demodulating a
second spread spectrum signal of said multiple spread spectrum
signals in accordance with a fixed time interval difference with
respect to said first arrival time.
14. The apparatus of claim 13 wherein said step of demodulating
said first spread spectrum signal comprises: means for descrambling
said first spread spectrum signal in accordance with a pseudorandom
noise sequence; means for extracting a pilot signal from said
pseudorandom noise descrambled signal; means for multiplying said
pseudorandom noise descrambled signal with said pilot signal; and
means for multiplying said phase adjusted signal by an orthogonal
channel sequence.
15. An apparatus for receiving multiple spread spectrum signals
each traveling upon a different propagation path and each having a
resultant arrival time difference with respect to one another, said
apparatus comprising: processing unit adapted for implementing
computer-readable instructions and a memory storage device adapted
for storing: a first set of computer-readable instructions for
demodulating a first spread spectrum signal of said multiple spread
spectrum signals in accordance with a first arrival time; and a
second set of computer-readable instructions for demodulating a
second spread spectrum signal of said multiple spread spectrum
signals in accordance with a fixed time interval difference with
respect to said first arrival time.
Description
CLAIM OF PRIORITY UNDER 35 U.S.C. .sctn.120
[0001] The present Application for Patent is a Continuation and
claims priority to patent application Ser. No. 09/182,367 entitled
"Method And Apparatus For Multipath Demodulation In A Code Division
Multiple Access communication System," filed Oct. 27, 1998, now
allowed, and assigned to the assignee hereof.
BACKGROUND
[0002] I. Field
[0003] The present invention relates to communications. More
particularly, the present invention relates to a novel and improved
method and apparatus for demodulating code division multiple access
(CDMA) signals.
[0004] II. Description of the Related Art
[0005] In a wireless radiotelephone communication system, many
users communicate over a wireless channel to connect to wireline
telephone systems. Communication over the wireless channel can be
one of a variety of multiple access techniques that allow a large
number of users in a limited frequency spectrum. These multiple
access techniques include time division multiple access (TDMA),
frequency division multiple access (FDMA), and code division
multiple access (CDMA).
[0006] The CDMA technique has many advantages. An exemplary CDMA
system is described in U.S. Pat. No. 4,901,307, entitled "SPREAD
SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR
TERRESTRIAL REPEATERS," assigned to the assignee of the present
invention.
[0007] In the '307 patent, a multiple access technique is disclosed
where a large number of mobile telephone system users, each having
a transceiver, communicate through satellite repeaters or
terrestrial base stations using CDMA spread spectrum communication
signals. The base station-to-mobile station signal transmission
path is referred to as the forward link and the mobile
station-to-base station signal transmission path is referred to as
the reverse link.
[0008] In using CDMA communications, the frequency spectrum can be
reused multiple times thus permitting an increase in system user
capacity. Each base station provides coverage to a limited
geographic area and links the mobile stations in its coverage area
through a cellular system switch to the public switched telephone
network (PSTN). When a mobile station moves to the coverage area of
a new base station, the routing of that user's call is transferred
to the new base station.
[0009] The CDMA modulation techniques discussed in the '307 patent
and in U.S. Pat. No. 5,102,459 entitled "SYSTEM AND METHOD FOR
GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,"
assigned to the assignee of the present invention, mitigate the
special problems of the terrestrial channel, such as multipath and
fading. Instead of being an impediment to system performance, as it
is with narrowband systems, separable multipaths can be diversity
combined in a mobile rake receiver for enhanced modem performance.
The use of a RAKE receiver for improved reception of CDMA signals
is disclosed in U.S. Pat. No. 5,109,390, entitled "DIVERSITY
RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM," assigned to the
assignee of the present invention. In the mobile radio channel,
multipath is created by reflection of the signal from obstacles in
the environment, such as buildings, trees, cars, and people. In
general the mobile radio channel is a time varying multipath
channel due to the relative motion of the structures that create
the multipath. For example, if an ideal impulse is transmitted over
the time varying multipath channel, the received stream of pulses
would change in time location, attenuation, and phase as a function
of the time that the ideal impulse was transmitted.
[0010] The multipath properties of the terrestrial channel produce,
at the receiver, signals having traveled several distinct
propagation paths. One characteristic of a multipath channel is the
time spread introduced in a signal that is transmitted through the
channel. As described in the '390 patent, the spread spectrum
pseudonoise (PN) modulation used in a CDMA system allows different
propagation paths of the same signal to be distinguished and
combined, provided the difference in path delays exceeds the PN
chip duration. If a PN chip rate of approximately 1 MHz is used in
a CDMA system, the full spread spectrum processing gain, equal to
the ratio of the spread bandwidth to the system data rate, can be
employed against paths having delays that differ by more than one
microsecond. A one microsecond path delay differential corresponds
to a differential path distance of approximately 300 meters.
[0011] Another characteristic of the multipath channel is that each
path through the channel may cause a different attenuation factor.
For example, if an ideal impulse is transmitted over a multipath
channel, each pulse of the received stream of pulses generally has
a different signal strength than other received pulses.
[0012] Yet another characteristic of the multipath channel is that
each path through the channel may cause a different phase on the
signal. If, for example, an ideal impulse is transmitted over a
multipath channel, each pulse of the received stream of pulses
generally has a different phase than other received pulses. This
can result in signal fading.
[0013] A fade occurs when multipath vectors are added
destructively, yielding a received signal that is smaller than
either individual vector. For example, if a sine wave is
transmitted through a multipath channel having two paths where the
first path has an attenuation factor of X dB, a time delay of d
with a phase shift of Q radians, and the second path has an
attenuation factor of X dB, a time delay of d with a phase shift of
Q+.pi. radians, no signal would be received at the
[0014] As described above, in current CDMA demodulator structures,
the PN chip interval defines the minimum separation two paths must
have in order to be combined. Before the distinct paths can be
demodulated, the relative arrival times (or offsets) of the paths
in the received signal must first be determined. The demodulator
performs this function by "searching" through a sequence of offsets
and measuring the energy received at each offset. If the energy
associated with a potential offset exceeds a certain threshold, a
demodulation element, or "finger" may be assigned to that offset.
The signal present at that path offset can then be summed with the
contributions of other fingers at their respective offsets.
[0015] A method and apparatus of finger assignment based on
searcher and finger energy levels is disclosed in U.S. Pat. No.
5,490,165, entitled "FINGER ASSIGNMENT IN A SYSTEM CAPABLE OF
RECEIVING MULTIPLE SIGNALS," assigned to the assignee of the
present invention. In the exemplary embodiment, the CDMA signals
are transmitted in accordance with the Telecommunications Industry
Association TIA/EIA/IS-95-A, entitled "MOBILE STATION-BASE STATION
COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM
CELLULAR SYSTEM." An exemplary embodiment of the circuitry capable
of demodulating IS-95 forward link signals is described in detail
in U.S. Pat. No. 5,764,592, entitled "MOBILE DEMODULATOR
ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS SYSTEM,"
assigned to the assignee of the present invention. An exemplary
embodiment of the circuitry capable of demodulating IS-95 reverse
link signals is described in detail in U.S. Pat. No. 5,654,979,
entitled "CELL SITE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM
MULTIPLE ACCESS COMMUNICATION SYSTEM," assigned to the assignee of
the present invention.
[0016] FIG. 1 shows an exemplary set of signals from a base station
arriving at the mobile station. It will be understood by one
skilled in the art that FIG. 1 is equally applicable to the signals
from a mobile station arriving at the base station. The vertical
axis represents the power received on a decibel (dB) scale. The
horizontal axis represents the delay in the arrival time of a
signal due to multipath delays. The axis (not shown) going into the
page represents a segment of time. The signals in the common plane
traveled along different paths arriving at the receiver at the same
time, but having been transmitted at different times.
[0017] In a common plane, peaks to the right were transmitted at an
earlier time by the base station than peaks to the left. For
example, the left-most peak spike 2 corresponds to the most
recently transmitted signal. Each signal spike 2-7 has traveled a
different path and therefore exhibits a different time delay and a
different amplitude response.
[0018] The six different signal spikes represented by spikes 2-7
are representative of a severe multipath environment. Typical urban
environments produce fewer usable paths. The noise floor of the
system is represented by the peaks and dips having lower energy
levels.
[0019] The task of the searcher is to identify the delay as
measured by the horizontal axis of signal spikes 2-7 for potential
finger assignment. The task of the finger is to demodulate one of a
set of the multipath peaks for combination into a single output. It
is also the task of a finger, once assigned to a multipath peak, to
track that peak as it may move in time.
[0020] The horizontal axis can also be thought of as having units
of PN offset. At any given time, the mobile station receives a
variety of signals from a base station, each of which has traveled
a different path and may have a different delay than the others.
The base station's signal is modulated by a PN sequence. A local
copy of the PN sequence is also generated at the mobile station.
Also at the mobile station, each multipath signal is individually
demodulated with a PN sequence code aligned to its received time
offset. The horizontal axis coordinates can be thought of as
corresponding to the PN sequence code offset that would be used to
demodulate a signal at that coordinate.
[0021] Note that each of the multipath peaks varies in amplitude as
a function of time, as shown by the uneven ridge of each multipath
peak. In the limited time shown, there are no major changes in the
multipath peaks. Over a more extended time range, multipath peaks
disappear and new paths are created as time progresses. The peaks
can also slide to earlier or later offsets as the path distances
change when the mobile station moves relative to the base station.
Each finger tracks these small variations in the signal assigned to
it.
[0022] In narrowband systems, the existence of multipath in the
radio channel can result in severe fading across the narrow
frequency band being used. Such systems are capacity constrained by
the extra transmit power needed to overcome a deep fade. As noted
above, CDMA signal paths may be discriminated and diversity
combined in the demodulation process.
[0023] Three major types of diversity exist: time diversity,
frequency diversity, and space/path diversity. Time diversity can
best be obtained by the use of repetition, time interleaving, and
error correction and detection coding that introduce redundancy. A
system may employ each of these techniques as a form of time
diversity.
[0024] CDMA, by its inherent wideband nature, offers a form of
frequency diversity by spreading the signal energy over a wide
bandwidth. The frequency selective fading that can cause a deep
fade across a narrowband system's frequency bandwidth usually only
affects a fraction of the frequency band employed by the CDMA
spread spectrum signal.
[0025] The rake receiver provides path diversity through its
ability to combine multipath delayed signals; all paths that have a
finger assigned to them must fade together before the combined
signal is degraded. Additional path diversity is obtained through a
process known as "soft hand-off" in which multiple simultaneous,
redundant links from two or more base stations can be established
with the mobile station. This supports a robust link in the
challenging environment at the cell boundary region. Examples of
path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled
"SOFT HAND-OFF IN A CDMA CELLULAR TELEPHONE SYSTEM," and U.S. Pat.
No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR
TELEPHONE SYSTEM," both assigned to the assignee of the present
invention.
[0026] Both the cross-correlation between different PN sequences
and the autocorrelation of a PN sequence, for all time shifts other
than zero, have a nearly zero average value. This allows the
different signals to be discriminated upon reception.
Autocorrelation and cross-correlation require that logical "0" take
on a value of "1" and logical "1" take on a value of "-1", or a
similar mapping, in order that a zero average value be
obtained.
[0027] However, such PN signals are not orthogonal. Although the
cross-correlation essentially averages to zero over the entire
sequence length for a short time interval, such as an information
bit time, the cross-correlation is a random variable with a
binomial distribution. As such, the signals interfere with each
other in much the same manner as if they were wide bandwidth
Gaussian noise at the same power spectral density.
[0028] It is well known in the art that a set of n orthogonal
binary sequences, each of length n, for n any power of 2 can be
constructed (see Digital Communications with Space Applications, S.
W. Golomb et al., Prentice-Hall, Inc., 1964, pp. 45-64). In fact,
orthogonal binary sequence sets are also known for most lengths
that are multiples of four and less than two hundred. One class of
such sequences that is easy to generate is called the Walsh
function; a Walsh function of order n can be defined recursively as
follows: 1 W ( n ) = W ( n / 2 ) W ( n / 2 ) W ( n / 2 ) W ( n / 2
) ( 1 )
[0029] where W' denotes the logical complement of W, and
W(1)=.vertline.0.vertline..
[0030] A Walsh sequence or code is one of the rows of a Walsh
function matrix. A Walsh function matrix of order n contains n
sequences, each of length n Walsh chips. A Walsh function matrix of
order n (as well as other orthogonal functions of length n) has the
property that over the interval of n bits, the cross-correlation
between all the different sequences within the set is zero. Every
sequence in the set differs from every other sequence in exactly
half of its bits. It should also be noted that there is always one
sequence containing all zeroes and that all the other sequences
contain half ones and half zeroes.
[0031] In the system described in the '459 patent, the call signal
begins as a 9600 bit per second information source which is then
converted by a rate 1/2 forward error correction encoder to a
19,200 symbols per second output stream. Each call signal broadcast
from a cell is covered with one of sixty-four orthogonal Walsh
sequences, each sixty-four Walsh chips, or one symbol, in duration.
Regardless of the symbol being covered, the orthogonality of all
Walsh sequences ensures that all interference from other user
signals in that cell are canceled out during symbol integration.
The non-orthogonal interference from other cells limits capacity on
the forward link.
[0032] Each base station in a CDMA system transmits in the same
frequency band using the same PN sequence, but with a unique offset
relative to an unshifted PN sequence aligned to a universal time
reference. The PN spreading rate is the same as the Walsh cover
rate, 1.2288 MHz, or 64 PN chips per symbol. In the preferred
embodiment, each base station transmits a pilot reference. In the
description of the present invention different information is
transmitted on the I and Q channels which increases the capacity of
the system.
[0033] The pilot channel is a "beacon" transmitting a constant zero
symbol and spread with the same I and Q PN sequences used by the
traffic bearing signals. In the exemplary embodiment, the pilot
channel is covered with the all zero Walsh sequence 0. During
initial system acquisition the mobile searches all possible shifts
of the PN sequence and once it has found a base station's pilot, it
can then synchronize itself to system time. As detailed below, the
pilot plays a fundamental role in the mobile demodulator rake
receiver architecture well beyond its use in initial
synchronization.
[0034] FIG. 2 depicts a generic rake receiver demodulator 10 for
receiving and demodulating the forward link signal 20 arriving at
the antenna 18. The analog transmitter and receiver 16 contain a
QPSK downconverter chain that outputs digitized I and Q channel
samples 32 at baseband. The sampling clock, CHIPX8 40, used to
digitize the receive waveform, is derived from a voltage controlled
temperature compensated local oscillator (TCXO).
[0035] The demodulator 10 is supervised by a microprocessor 30
through the databus 34. Within the demodulator, the I and Q samples
32 are provided to a plurality of fingers 12a-c and a searcher 14.
The searcher 14 searches out windows of offsets likely to contain
multipath signal peaks suitable for assignment of fingers 12a-c.
For each offset in the search window, the searcher 14 reports the
pilot energy it found at that offset to the microprocessor. The
fingers 12a-c are then surveyed, and those unassigned or tracking
weaker paths are assigned by the microprocessor 30 to offsets
containing stronger paths identified by searcher 14.
[0036] Once a finger 12a-c has locked onto the multipath signal at
its assigned offset it then tracks that path on its own until the
path fades away or until it is reassigned using its internal time
tracking loop. This finger time tracking loop measures energy on
either side of the peak at the offset at which the finger is
currently demodulating. The difference between these energies forms
a metric which is then filtered and integrated.
[0037] The output of the integrator controls a decimator that
selects one of the input samples over a chip interval to use in
demodulation. If a peak moves, the finger adjusts its decimator
position to move with it. The decimated sample stream is then
despread with the PN sequence consistent with the offset to which
the finger is assigned. The despread I and Q samples are summed
over a symbol to produce a pilot vector (P.sub.I, P.sub.Q). These
same despread I and Q samples are Walsh uncovered using the Walsh
code assignment unique to the mobile user and the uncovered,
despread I and Q samples are summed over a symbol to produce a
symbol data vector (D.sub.I, D.sub.Q). The dot product operator is
defined as
P(n).multidot.D(n)=P.sub.I(n)D.sub.I(n)+P.sub.Q(n)D.sub.Q(n),
(2)
[0038] where P.sub.I(n) and P.sub.Q(n) are respectively the I and Q
components of the pilot vector P for symbol n and D.sub.I(n) and
D.sub.Q(n) are respectively the I and Q components of the data
vector D for symbol n.
[0039] Since the pilot signal vector is much stronger than the data
signal vector it can be used as an accurate phase reference for
coherent demodulation; the dot product computes the magnitude of
the data vector component in phase with the pilot vector. As
described in U.S. Pat. No. 5,506,865, entitled "PILOT CARRIER DOT
PRODUCT CIRCUIT" and assigned to the assignee of the present
invention, the dot product weights the finger contributions for
efficient combining, in effect scaling each finger symbol output
42a-c by the relative strength of the pilot being received by that
finger. Thus the dot product performs the dual role of both phase
projection and finger symbol weighting needed in a coherent rake
receiver demodulator.
[0040] Each finger has a lock detector circuit that masks the
symbol output to the combiner 42 if its long term average energy
does not exceed a minimum threshold. This ensures that only fingers
tracking a reliable path will contribute to the combined output,
thus enhancing demodulator performance.
[0041] Due to the relative difference in arrival times of the paths
to which each finger 12a-c is assigned, each finger 12a-c has a
deskew buffer that aligns the finger symbol streams 42a-c so that
the symbol combiner 22 can sum them together to produce a "soft
decision" demodulated symbol. This symbol is weighted by the
confidence that it correctly identifies the originally transmitted
symbol. The symbols are sent to a deinterleaver/decoder circuit 28
that first frame deinterleaves and then forward error correction
decodes the symbol stream using the maximum likelihood Viterbi
algorithm. The decoded data is then made available to the
microprocessor 30 or to other components, such as a speech vocoder,
for further processing.
[0042] To demodulate correctly, a mechanism is needed to align the
local oscillator frequency with the clock used at the cell to
modulate the data. Each finger makes an estimate of the frequency
error by measuring the rotation rate of the pilot vector in QPSK I,
Q space using the cross product vector operator:
P(n).times.P(n-1)=P.sub.I(n)P.sub.Q(n-1)-P.sub.I(n-1)P.sub.Q(n)
(3)
[0043] The frequency error estimates from each finger 44a-c are
combined and integrated in frequency error combiner 26. The
integrator output, LO_ADJ 36, is then fed to the voltage control of
the TCXO in the analog transmitter and receiver 16 to adjust the
clock frequency of the CHIPX8 clock 40, thus providing a closed
loop mechanism for compensating for the frequency error of the
local oscillator.
[0044] As described above, in current demodulator structures, a
path must differ by at least one PN chip to have a separate finger
allocated to its demodulation. However, there are cases when paths
differ by less than a PN chip interval in the time, this situation
leads to the existence of a "fat path." Under traditional
demodulator implementations, only one finger could be allocated to
demodulate the fat path. One of the reasons for this is that once
assigned to a path, the finger tracks the path movement
independently. Without central coordination of the fingers multiple
fingers will converge to the same peak of the fat path. In
addition, the searcher tends to get confused when paths are tracked
which are to close to one another.
[0045] On an orthogonal forward link, there is a great deal of
energy in each of the paths because all of the energy from the base
station to all mobiles is transmitted using the same PN offset
which are channelized by use of orthogonal code sequences.
Moreover, orthogonal code sequences have poor autocorrelation in
that the correlation between orthogonal code sequences is high.
Thus, when paths on the forward link differ by less than a PN chip
interval, the signals cannot be distinguished from one another by
the outer PN spreading nor is the coding gain of the orthogonal
spreading realized because of the time shift. The energy of the
close multipath components in this case serves as noise and
substantially degrades the performance of the demodulator assigned
to the fat path. On the reverse link, close multipath components
can also cause degradation of the demodulator assigned to the fat
path.
[0046] The present invention is described with respect to the
improvement of the demodulation of the forward link. However, the
present invention is equally applicable to improving the
demodulation of the reverse link.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] The features, objects, and advantages of the present
invention will become more apparent from the detailed description
set forth below when taken in conjunction with the drawings in
which like reference characters identify correspondingly throughout
and wherein:
[0048] FIG. 1 represents an exemplary severe multipath signal
condition;
[0049] FIG. 2 is a block diagram of a current demodulation
system;
[0050] FIG. 3 is an illustration of a fat path situation in which a
number of multipath components are very closely spaced with respect
to one another in arrival times at the receiver;
[0051] FIG. 4 is an illustration of the receiver structure that
provides for effective demodulation of the closely spaced multipath
components;
[0052] FIG. 5 is an illustration of an improved demodulation
structure that allows for a single accumulator in the receiver
architecture;
[0053] FIG. 6 is a first embodiment of the fat path demodulator of
the present invention wherein four fingers are used to demodulate
four multipath components with arrival times offset from one
another by half of one PN chip wherein the method of path
discrimination is through the offsetting of the PN sequences with
accompanying deskewing prior to combination;
[0054] FIG. 7 is a second embodiment of the fat path demodulator of
the present invention wherein four fingers are used to demodulate
four multipath components with arrival times offset from one
another by half of one PN chip wherein the received signal is
delayed by varying amounts of times before being provided to the
respective fingers;
[0055] FIG. 8 is a third embodiment of the fat path demodulator of
the present invention wherein six fingers are used to demodulate
six multipath components with arrival times offset from one another
by half of one PN chip wherein the received signal is delayed by
varying amounts of times before being provided to the respective
fingers;
[0056] FIG. 9 is a fourth embodiment of the fat path demodulator of
the present invention wherein five fingers are used to demodulate
five multipath components with arrival times offset from one
another by one third of one PN chip wherein the received signal is
delayed by varying amounts of times before being provided to the
respective fingers;
[0057] FIG. 10 illustrates an alternative embodiment that allows
for elimination of the input switch;
[0058] FIG. 11 illustrates an alternative embodiment that allows
for eleimination of all but one Walsh sequence multiplier;
[0059] FIG. 12 illustrates an alternative embodiment that allows
for the elimination of two demodulator structures; and
[0060] FIG. 13 illustrates an alternative embodiment that allows
for the elimination of two demodulators and allows the reciver to
operate at the PN chip rate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0061] FIG. 3 shows an exemplary set of signals from a base station
arriving at the mobile station at a given time. It will be
understood by one skilled in the art that FIG. 3 is equally
applicable to the signals from a mobile station arriving at the
base station. The vertical axis represents the power received on a
decibel (dB) scale. The horizontal axis represents the delay in the
arrival time of a signal due to multipath delays. The signals on
the x-axis traveled along different paths arriving at the receiver
at the same time, but having been transmitted at different
times.
[0062] In a common plane, peaks to the right were transmitted at an
earlier time by the base station than peaks to the left. For
example, the left-most peak spike 50 corresponds to the most
recently transmitted signal. Each signal spike 50, 54 and 58 has
traveled a different path and therefore exhibits a different time
delay and a different amplitude response.
[0063] The three different signal spikes represented by spikes 50,
54 and 58 are representative of a severe multipath environment. As
described previously, the task of the searcher is to identify the
delay as measured by the horizontal axis of signal spikes 50, 54
and 58 for potential finger assignment. However, in the present
invention, the additional task of the searcher is to identify peak
54 as a fat path or set of multipath components to which the
demodulator structure of the present invention is capable of
effective demodulation of close multipath components. The task of
each of the finger is to demodulate one of a set of the multipath
peaks for combination into a single output. It is also the task of
a finger, once assigned to a multipath peak, to track that peak as
it may move in time.
[0064] FIG. 4 depicts the rake receiver demodulator 110 of the
present invention for receiving and demodulating the forward link
signal 120 arriving at the antenna 118 of the present invention.
The analog transmitter and receiver 116 contain a QPSK
downconverter chain that outputs digitized I and Q channel samples
132 at baseband. In an exemplary embodiment, the sampling clock,
CHIPX8 140, used to digitize the receive waveform, is derived from
a voltage controlled temperature compensated local oscillator
(TCXO).
[0065] The demodulator 110 is supervised by a microprocessor 130
through the databus 134. Within the demodulator, the I and Q
samples 132 are provided to a plurality of fingers 112a-c and a
searcher 114. Although the exemplary embodiment is described in
terms of QPSK demodulation, the present invention is equally
applicable to BPSK, QAM (Quadrature Amplitude Modulation), M-ary
PSK or any known modulation method. The searcher 114 searches out
windows of offsets likely to contain multipath signal peaks
suitable for assignment of fingers 112a-c. For each offset in the
search window, the searcher 114 reports the pilot energy it found
at that window of offsets to the microprocessor 130. In the present
invention, microprocessor 130 determines where to assign fingers
and determines whether and where to assign a fat path
demodulators.
[0066] Searcher 114 reports the energies in a window around peaks
50, 54 and 58. Microprocessor 130 determine from the reported
energies that peaks 50 and 58 were narrow and could be successfully
demodulated with a single path demodulator. Microprocessor 130
would also be able to identify the multipath component at peak 54
as a fat path and would assign for its demodulation the fat path
demodulator of the present invention. So for example, fingers 112a
and 112b demodulate single paths and are assigned to paths 50 and
58 of FIG. 3. Finger 112c, on the other hand, are directed by
microprocessor 130 to perform a fat path demodulation and would be
assigned to demodulate path 54.
[0067] FIG. 5 illustrates a novel RAKE receiver structure that uses
a single accumulator instead of an accumulator for each finger as
is provided in current RAKE receiver structures. The digitized
samples are provided to complex PN despreader 150 of demodulator
158. In the exemplary embodiment, the signals are complex PN spread
as described in U.S. patent application Ser. No. 08/856,428,
entitled "HIGH DATA RATE CDMA WIRELESS CMMUNICATION SYSTEM USING
VARIABLE SIZED CHANNEL CODES," filed May 14, 1997, assigned to the
assignee of the present invention, in accordance with the following
equations:
I=I'PN.sub.I+Q'PN.sub.Q (4)
Q=I'PN.sub.Q-Q'PN.sub.I. (5)
[0068] where PN.sub.I and PN.sub.Q are distinct PN spreading codes
and I' and Q' are two channels being spread at the transmitter.
Complex PN despreader 150 removes the complex spreading based on
the PN codes, PN.sub.I and PN.sub.Q, to provide two complex PN
despread signals.
[0069] The complex PN despread signals are provided to pilot filter
152 and complex conjugate multiplier 154. Pilot filter 152 uncovers
the pilot signal in accordance with the orthogonal covering
(W.sub.pilot) and, in a preferred embodiment, provides some
filtering to the resultant signal to remove the effects of noise on
the received signal. In the exemplary embodiment, the pilot signal
is covered using Walsh 0 which is the all zeroes Walsh sequence.
Thus, uncovering the Walsh sequence is a no op and pilot filter 152
simply acts as a low pass filter to reduce the effect of channel
noise.
[0070] Complex conjugate multiplier 154 multiplies the signal from
complex PN despreader 150 by the conjugate of the filtered pilot
signal from pilot filter 152. By multiplying the complex despread
data by the conjugate of the signals from pilot filter 152, the
demodulator removes any phase error from the received signal. In
effect complex conjugate multiply circuit projects the received
signal onto the pilot signal and outputs the magnitudes of the
projections.
[0071] The signals from complex conjugate multiplier 154 are
provided to Walsh multiplier 156. Walsh multiplier 156 multiply the
I and Q traffic channels with the orthogonal traffic channel
covering sequences W.sub.traffic. The traffic channel data is then
output to symbol combiner 160. Demodulators 158b and 158c
demodulate the received signals for different multipath components
using different PN offsets of PN.sub.I and PN.sub.Q and are
deskewed prior to being provided to combiner 160. In the exemplary
embodiment, only signals with energies exceeding a predetermined
threshold are combined in combiner 160. The combined symbol
energies are thereafter provided to accumulator 162 which
accumulates the combined energy valves over Walsh sequence
intervals to provided estimates of the I+Q values.
[0072] In an alternative embodiment, complex conjugate multiplier
154 and Walsh multiplier 156 can be interchanged without the need
for altering any other functions. It will be understood by one
skilled in the art that the simple rearrangements of elements are
well known in the art and are within the scope of the present
invention.
[0073] Before turning our attention to the implementation of the
fat finger demodulator structure, let us briefly examine the
process that allows combining of the received signals in a CDMA
communication system. Referring back to FIG. 1, it was described
earlier that the peaks on the common plane were transmitted at
different times and followed different propagation paths so as to
arrive at the receiver at a common time. As described above, the
signal in peak 2 corresponds to the most recently transmitted
signal. The signal in peak 3 was transmitted approximately 2 PN
chip intervals in time later. In order to combine the information
in peak 2 with the information in peak 3, the information from peak
2 must be delayed by two PN chip intervals before being combined
with the information from peak 3 so that different version of the
same information are combined.
[0074] The proposed fat path demodulators take advantage of both PN
shifting and time delay to provide for deskewing of the
information. Turning to FIG. 6, a fat path demodulator is
illustrated which performs demodulation of fat paths comprising a
set of closely spaced multipath components with energy spread
across a plurality of PN chips. In FIG. 6, four demodulators
200a-200d are provided to demodulate paths that are a fixed half PN
chip distance from one another. The demodulators move together
demodulating a PN offsets that are offset from one another by fixed
increments. In an alternative embodiment, the microprocessor in the
receiver could be used to determine the shape of the fat path and
would adjust the values of delay elements in accordance with the
shape of the path grouping. In the exemplary embodiment, one of the
demodulators is the master and tracks the peak of set of multipath
signals and the other demodulators act as slaves and follow the
tracking of the master demodulator.
[0075] Demodulator 200a and demodulator 200c demodulate the
received signal using PN sequences that are offset from one another
by one PN chip interval. This can be seen by observing that the
signal provided to input 198 is provided directly to demodulators
200a and 200c. Demodulator 200a demodulates the received signal in
accordance with a PN offset from PN generator 206 that is delayed
by one PN chip interval by delay element 208 and in accordance with
a Walsh sequence from Walsh generator 218 that is delayed by one PN
chip interval by delay element 216.
[0076] As described with respect to FIG. 1, the signal demodulated
by demodulator 200a followed a propagation path that took one PN
chip longer to traverse than the propagation path upon which the
signal demodulated by demodulator 200c followed. In order to
properly combine the information, delay element 220 delays the data
demodulated by demodulator 200c prior to combining with the
demodulated data from demodulator 200a.
[0077] The same combining operation is performed with respect to
the signals demodulated by demodulators 200b and 200d. Demodulators
200b and 200d demodulate signal that have traversed paths that
differ by one half PN chip interval from the signals demodulated by
demodulators 200a and 200c, respectively. The deskewing of the half
chip path difference is not performed by a delays but rather is
inherent in the performance of the signal combining operation in
combiner element 224 which combines when the demodulated data from
demodulators 200b and 200d is available to combiner element 224.
This intrinsically means that the information from demodulators
200a and 200c was provided a half PN cycle earlier than the
demodulated information from demodulators 200b and 200d in essence
performing the additional deskewing operation. One skilled in the
art will recognize that this operation could be performed by
placing addition 1/2 PN chip delays on the outputs of demodulators
200b and 200d.
[0078] Moving to the detailed operation of the demodulator of FIG.
6, baseband samples at twice the PN chip rate are provided to
switch 202. Switch 202 switches between outputs 198 and 199 at
twice the rate of the PN chip cycle. The first base band sample is
provided to demodulators 200a and 200c. The next baseband sample
that arrives one half PN chip interval later is provided to
demodulators 200b and 200d.
[0079] The first sample is provided through switch 202 on line 198
to demodulator 200a. The sample is PN descrambled in PN
descrambling element 204a. In the exemplary embodiment, PN
descrambling element 204a descrambles the sample in accordance with
two PN sequences (PN.sub.I and PN.sub.Q) provided by PN generator
206. The PN sequences are delayed by delay element 208 by one PN
chip period. The complex despreading operation is performed as
described above with respect to complex despreading element 150 of
FIG. 5.
[0080] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 212a and to pilot filter
210a. Complex conjugate multiplier removes phase ambiguities that
are introduced by the propagation path. Pilot filter 210a uncovers
the pilot channel in accordance with the Walsh covering for the
pilot channel W.sub.pilot. In the exemplary embodiment, W.sub.pilot
is the all zeroes Walsh sequence for which the uncovering operation
is a No Op. In this special case, pilot filter 210a is simply a low
pass filter which the noise from the pilot signal. The complex
conjugate of the filtered pilot signal and the complex PN despread
sequences are multiplied in complex conjugate multiplier 212a which
computes the dot product of the pilot channel and the PN
descrambled sequence to provide a scalar sequence to Walsh sequence
multiplier 214a.
[0081] Walsh sequence multiplier 214a multiplies the input scalar
sequence from complex conjugate multiplier 212a by the traffic
channel Walsh sequence from Walsh generator 218 which is delayed by
one PN chip interval by delay element 216. The multiplied sequence
is then provided to combiner element 224.
[0082] The first sample is redundantly provided through switch 202
on line 198 to demodulator 200c. The sample is PN descrambled in PN
descrambling element 204c. In the exemplary embodiment, PN
descrambling element 204c descrambles the sample in accordance with
two PN sequences (PN.sub.I and PN.sub.Q) provided by PN generator
206. The PN sequences are provided directly to PN descrambling
element 204c which results in the sample being demodulated by a PN
sequence offset from the sequence used by demodulator 200a by one
PN chip interval.
[0083] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 212c and to pilot filter
210c. Pilot filter 210c uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, W.sub.pilot is the all zeroes Walsh sequence
for which the uncovering operation is a No Op. In this special
case, pilot filter 210c is simply a low pass filter which removes
the noise from the pilot signal. The complex conjugate of the
filtered pilot signal and the complex PN despread sequences are
multiplied in complex conjugate multiplier 212c which computes the
dot product of the pilot channel and the PN descrambled sequence to
provide a scalar sequence to Walsh sequence multiplier 214c.
[0084] Walsh sequence multiplier 214c multiplies the input scalar
sequence from complex conjugate multiplier 212c by the traffic
channel Walsh sequence from Walsh generator 218. The multiplied
sequence is then provided to summing means 224.
[0085] After one half PN chip interval, switch 202 toggles so as to
put the next sample, received one half PN chip interval later, on
input line 199 to demodulators 200b and 200d. Within demodulator
200b, the sample is PN descrambled in PN descrambling element 204b.
As described previously PN descrambling element 204b descrambles
the sample in accordance with two PN sequences (PN.sub.I and
PN.sub.Q) provided by PN generator 206b. The PN sequences are
delayed by delay element 208 by one PN chip period.
[0086] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 212b and to pilot filter
210b. Pilot filter 210b uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, pilot filter 210b is simply a low pass filter
which removes the noise from the pilot signal. The complex
conjugate of the filtered pilot signal and the complex PN despread
sequences are multiplied in complex conjugate multiplier 212b which
computes the dot product of the pilot channel and the PN
descrambled sequence to provide a scalar sequence to Walsh sequence
multiplier 214b.
[0087] Walsh sequence multiplier 214b multiplies the input scalar
sequence from complex conjugate multiplier 212b by the traffic
channel Walsh sequence from Walsh generator 218 which is delayed by
one PN chip interval by delay element 216. The multiplied sequence
is then provided to combiner means 224.
[0088] The second sample is redundantly provided through switch 202
on line 199 to demodulator 200d. The second sample is PN
descrambled in PN descrambling element 204d. In the exemplary
embodiment, PN descrambling element 204d descrambles the sample in
accordance with two PN sequences (PN.sub.I and PN.sub.Q) provided
by PN generator 206. The PN sequences are provided directly to PN
descrambling element 204d which results in the sample being
demodulated by a PN sequence offset from the sequence used by
demodulator 200b by one PN chip interval.
[0089] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 212d and to pilot filter
210d. Pilot filter 210c uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, pilot filter 210d is simply a low pass filter
that removes the noise from the pilot signal. The complex conjugate
of the filtered pilot signal and the complex PN despread sequences
are multiplied in complex conjugate multiplier 212d which computes
the dot product of the pilot channel conjugate and the PN
descrambled sequence to provide a scalar sequence to Walsh sequence
multiplier 214d.
[0090] Walsh sequence multiplier 214d multiplies the input scalar
sequence from complex conjugate multiplier 212d by a Walsh sequence
from Walsh generator 218. The multiplied sequence is then provided
to combiner 224.
[0091] After the demodulated signals from demodulators 200b and
200d have been provided to combiner element 224, combiner element
224 combines the energies and outputs the combined energy values to
accumulator 226. Accumulator 226 performs the integration or
summation of the input symbols over the Walsh symbol interval.
Combiner element 224 can perform the combination in a variety of
ways. Combiner element 224 could sum only demodulated data with
energy above a threshold value or could sum all of the energies.
Alternatively, combiner 224 could select the demodulated data with
the greatest energy. In an alternative embodiment, combiner 224
combines the energy based on the power of the demodulated pilot
from pilot filter 210. For the sake of clarity, optional lines from
pilot filters 210 to combiner 224 have been omitted.
[0092] In FIG. 7, a second fat path demodulator is illustrated
where the delays are applied to the input signal instead of the
demodulation elements. In FIG. 7, four demodulators 300a-300d are
provided to demodulate paths that are a fixed half PN chip distance
from one another. The demodulators move together demodulating PN
offsets that are offset from one another by fixed increments. As
described previously a microprocessor could be used to vary the
amount of delay provided by delay elements 320 and 322. In the
exemplary embodiment, one of the demodulators is the master and
tracks the peak of set of multipath signals and the other
demodulators act as slaves and follow the master demodulator. In
the exemplary embodiment, a metric such as the power from pilot
filter 210 can be used by the master finger to track the movement
of the peak.
[0093] Demodulator 300a and demodulator 300c demodulate the
received signal that are delayed with respect to one another by one
PN chip interval. The signal provided to input 298 is provided
directly to demodulator 300a. The signal is delayed by one PN chip
interval by delay element 320 prior to being provided to
demodulator 300c. If a first version of the transmitted signal
traverses a first propagation path to be successfully demodulated
by demodulator 300a, then a second version of the transmitted
signal would need to traverse a second propagation path requiring a
PN chip interval longer than the time required to traverse the
first propagation path in order to be successfully demodulated by
demodulator 300c.
[0094] Half of a PN chip interval later, switch 302 toggles to
provide the sample taken half a chip interval later onto line 299.
The second sample is provided directly to demodulator 300b and
delayed by one PN chip interval by delay element 322 prior to being
provided to demodulator 300d. This performs the path diversity
combination as described above with respect to demodulators 300a
and 300c.
[0095] The first sample is provided through switch 302 on line 298
to demodulator 300a. The sample is PN descrambled in PN
descrambling element 304a. In the exemplary embodiment, PN
descrambling element 204a descrambles the sample in accordance with
two PN sequences (PN.sub.I and PN.sub.Q) provided by PN generator
206. The complex despreading operation is performed as described
above with respect to complex despreading element 150a.
[0096] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 312a and to pilot filter
310a. Complex conjugate multiplier removes phase ambiguities that
are introduced by the propagation path. Pilot filter 310a uncovers
the pilot channel in accordance with the Walsh covering for the
pilot channel W.sub.pilot. In the exemplary embodiment, W.sub.pilot
is the all zeroes Walsh sequence for which the uncovering operation
is a No Op. In this special case, pilot filter 310a is simply a low
pass filter which removes the noise from the pilot signal. The
complex conjugate of the filtered pilot signal and the complex PN
despread sequences are multiplied in complex conjugate multiplier
312a which computes the dot product of the pilot channel conjugate
and the PN descrambled sequence to provide a scalar sequence to
Walsh sequence multiplier 314a.
[0097] Walsh sequence multiplier 314a multiplies the input scalar
sequence from complex conjugate multiplier 312a by the Walsh
traffic sequence from Walsh generator 318. The multiplied sequence
is then provided to combiner element 224.
[0098] The first sample is redundantly provided through switch 302
on line 298 to delay element 320. Delay element 320 delays the
signal by one PN chip interval prior to providing the sample to
demodulator 300c. Thus, the signal successfully demodulated by
demodulator 300c will have traversed a propagation path that
required one PN chip less time to traverse than the path that was
successfully demodulated by demodulator 300a. The sample is PN
descrambled in PN descrambling element 304c. In the exemplary
embodiment, PN descrambling element 304c descrambles the sample in
accordance with two PN sequences (PN.sub.I and PN.sub.Q) provided
by PN generator 306.
[0099] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 312c and to pilot filter
310c. Pilot filter 310c uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, W.sub.pilot is the all zeroes Walsh sequence
for which the uncovering operation is a No Op. In this special
case, pilot filter 310c is simply a low pass filter which removes
the noise from the pilot signal. The complex conjugate of the
filtered pilot signal and the complex PN despread sequences are
multiplied in complex conjugate multiplier 212c which computes the
dot product of the pilot channel conjugate and the PN descrambled
sequence to provide a scalar sequence to Walsh sequence multiplier
214c.
[0100] Walsh sequence multiplier 314c multiplies the input scalar
sequence from complex conjugate multiplier 312c by the Walsh
traffic sequence from Walsh generator 318. The multiplied sequence
is then provided to combiner element 324.
[0101] After one half PN chip interval, switch 302 toggles so as to
put the next sample, received one half PN chip interval later, on
input line 299 to demodulators 300b and 300d. Within demodulator
300b, the sample is PN descrambled in PN descrambling element 304b.
In the exemplary embodiment, PN descrambling element 304b
descrambles the sample in accordance with two PN sequences
(PN.sub.I and PN.sub.Q) provided by PN generator 306b.
[0102] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 312b and to pilot filter
310b. Pilot filter 310b uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, pilot filter 310b is simply a low pass filter
which removes the noise from the pilot signal. The complex
conjugate of the filtered pilot signal and the complex PN despread
sequences are multiplied in complex conjugate multiplier 312b which
computes the dot product of the pilot channel conjugate and the PN
descrambled sequence to provide a scalar sequence to Walsh sequence
multiplier 314b.
[0103] Walsh sequence multiplier 314b multiplies the input scalar
sequence from complex conjugate multiplier 312b by the Walsh
traffic sequence from Walsh generator 318 which is delayed by one
PN chip interval by delay element 316. The multiplied sequence is
then provided to combiner element 324.
[0104] The second sample is redundantly provided through switch 302
on line 299 to delay element 322. Delay element 322 delays the
signal by one PN chip interval prior to providing it to demodulator
300d. Demodulator 300d successfully demodulates a signals that
traversed a path that took one PN chip less time to traverse than
the path successfully demodulated by demodulator 300b. The second
sample is PN descrambled in PN descrambling element 304d. In the
exemplary embodiment, PN descrambling element 304d descrambles the
sample in accordance with two PN sequences (PN.sub.I and PN.sub.Q)
provided by PN generator 306.
[0105] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 312d and to pilot filter
310d. Pilot filter 310d uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, pilot filter 310d is simply a low pass filter
that removes the noise from the pilot signal. The complex conjugate
of the filtered pilot signal and the complex PN despread sequences
are multiplied in complex conjugate multiplier 312d which computes
the dot product of the pilot channel conjugate and the PN
descrambled sequence to provide a scalar sequence to Walsh sequence
multiplier 314d.
[0106] Walsh sequence multiplier 314d multiplies the input scalar
sequence from complex conjugate multiplier 312d by the Walsh
traffic sequence from Walsh generator 318. The multiplied sequence
is then provided to combiner element 324.
[0107] After the demodulated signals from demodulators 300b and
300d have been provided to combiner element 324, combiner element
324 combines the energies and outputs the combined energy values to
accumulator 326. As described earlier, the combining operation can
take many forms all of which are within the scope of the present
invention. Accumulator 326 performs the integration or summation of
the input symbols over the Walsh symbol interval.
[0108] Turning to FIG. 8. demodulators 400a, 400c and 400e
demodulate the received signal delayed with respect to one another
by one PN chip interval. Similarly, demodulators 400b, 400d and
400f demodulate a second set of samples delayed with respect to one
another by one PN chip interval. The theory behind the operation of
the demodulator of FIG. 8 is identical to that of FIG. 7. Each
demodulator demodulates the signal that traversed a path that
required a time to traverse differing from one another by one half
PN chip interval.
[0109] The first sample is provided through switch 402 onto line
398 to demodulator 400a. The sample is PN descrambled in PN
descrambling element 404a. In the exemplary embodiment, PN
descrambling element 404a descrambles the sample in accordance with
two PN sequences (PN.sub.I and PN.sub.Q) provided by PN generator
406 as described previously.
[0110] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 412a and to pilot filter
410a. Complex conjugate multiplier 412a removes phase ambiguities
that are introduced by the propagation path. Pilot filter 410a
uncovers the pilot channel in accordance with the Walsh covering
for the pilot channel W.sub.pilot. In the exemplary embodiment,
W.sub.pilot, is the all zeroes Walsh sequence for which the
uncovering operation is a No Op. In this special case, pilot filter
410a is simply a low pass filter which removes the noise from the
pilot signal. The complex conjugate of the filtered pilot signal
and the complex PN despread sequences are multiplied in complex
conjugate multiplier 412a which computes the dot product of the
pilot channel conjugate and the PN descrambled sequence to provide
a scalar sequence to Walsh sequence multiplier 414a.
[0111] Walsh sequence multiplier 414a multiplies the input scalar
sequence from complex conjugate multiplier 412a by the Walsh
traffic sequence from Walsh generator 418. The multiplied sequence
is then provided to combiner element 424.
[0112] The first sample is redundantly provided through switch 402
on line 398 to delay element 420. Delay element 420 delays the
signal by one PN chip interval prior to providing the sample to
demodulator 400c. Thus, the signal successfully demodulated by
demodulator 400c will have traversed a propagation path that
required one PN chip less time to traverse than the path that was
successfully demodulated by demodulator 400a. The sample is PN
descrambled in PN descrambling element 404c. In the exemplary
embodiment, PN descrambling element 404c descrambles the sample in
accordance with two PN sequences (PN.sub.I and PN.sub.Q) provided
by PN generator 406.
[0113] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 412c and to pilot filter
410c. Pilot filter 410c uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, W.sub.pilot is the all zeroes Walsh sequence
for which the uncovering operation is a No Op. In this special
case, pilot filter 410c is simply a low pass filter which removes
the noise from the pilot signal. The complex conjugate of the
filtered pilot signal and the complex PN despread sequences are
multiplied in complex conjugate multiplier 412c which computes the
dot product of the pilot channel conjugate and the PN descrambled
sequence to provide a scalar sequence to Walsh sequence multiplier
414c.
[0114] Walsh sequence multiplier 414c multiplies the input scalar
sequence from complex conjugate multiplier 412c by Walsh traffic
sequence from Walsh generator 418. The multiplied sequence is then
provided to combiner element 424.
[0115] The first sample is also redundantly provided through delay
element 422a to delay element 422b. Delay element 422b delays the
signal by one additional PN chip interval prior to providing the
sample to demodulator 400e. Thus, the signal successfully
demodulated by demodulator 400e will have traversed a propagation
path that required one PN chip less time to traverse than the path
that was successfully demodulated by demodulator 400c and two PN
chip intervals less that the signal successfully demodulated by
demodulator 400a. The sample is PN descrambled in PN descrambling
element 404e. In the exemplary embodiment, PN descrambling element
404e descrambles the sample in accordance with two PN sequences
(PN.sub.I and PN.sub.Q) provided by PN generator 406.
[0116] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 412e and to pilot filter
410e. Pilot filter 410e uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, W.sub.pilot is the all zeroes Walsh sequence
for which the uncovering operation is a No Op. In this special
case, pilot filter 410e is simply a low pass filter which removes
the noise from the pilot signal. The complex conjugate of the
filtered pilot signal and the complex PN despread sequences are
multiplied in complex conjugate multiplier 412e which computes the
dot product of the pilot channel conjugate and the PN descrambled
sequence to provide a scalar sequence to Walsh sequence multiplier
414e.
[0117] Walsh sequence multiplier 414e multiplies the input scalar
sequence from complex conjugate multiplier 412e by the Walsh
traffic sequence from Walsh generator 418. The multiplied sequence
is then provided to combiner element 424.
[0118] After one half PN chip interval, switch 402 toggles so as to
put the next sample, received one half PN chip interval later, on
input line 399 to demodulators 400b, 400d and 400f.
[0119] Within demodulator 400b, the sample is PN descrambled in PN
descrambling element 404b. In the exemplary embodiment, PN
descrambling element 404b descrambles the sample in accordance with
two PN sequences (PN.sub.I and PN.sub.Q) provided by PN generator
406b. The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 412b and to pilot filter
410b. Pilot filter 410b uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, pilot filter 410b is simply a low pass filter
which removes the noise from the pilot signal. The complex
conjugate of the filtered pilot signal and the complex PN despread
sequences are multiplied in complex conjugate multiplier 412b which
computes the dot product of the pilot channel conjugate and the PN
descrambled sequence to provide a scalar sequence to Walsh sequence
multiplier 414b.
[0120] Walsh sequence multiplier 414b multiplies the input scalar
sequence from complex conjugate multiplier 412b by Walsh traffic
sequence from Walsh generator 418. The multiplied sequence is then
provided to combiner 424.
[0121] The second sample is redundantly provided through switch 402
on line 399 to delay element 420a. Delay element 420a delays the
signal by one PN chip interval prior to providing it to demodulator
400d. Demodulator 400d successfully demodulates a signals that
traversed a path that took one PN chip less time to traverse than
the path successfully demodulated by demodulator 400b. The second
sample is PN descrambled in PN descrambling element 404d. In the
exemplary embodiment, PN descrambling element 404d descrambles the
sample in accordance with two PN sequences (PN.sub.I and PN.sub.Q)
provided by PN generator 406.
[0122] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 412d and to pilot filter
410d. Pilot filter 410d uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, pilot filter 410d is simply a low pass filter
that removes the noise from the pilot signal. The complex conjugate
of the filtered pilot signal and the complex PN despread sequences
are multiplied in complex conjugate multiplier 412d which computes
the dot product of the pilot channel conjugate and the PN
descrambled sequence to provide a scalar sequence to Walsh sequence
multiplier 414d.
[0123] Walsh sequence multiplier 414d multiplies the input scalar
sequence from complex conjugate multiplier 412d by a Walsh sequence
from Walsh generator 418. The multiplied sequence is then provided
to combiner 424.
[0124] The second sample is redundantly provided through delay
element 420a to delay element 420b. Delay element 420b delays the
signal by one additional PN chip interval prior to providing the
sample to demodulator 400f. Thus, the signal successfully
demodulated by demodulator 400f will have traversed a propagation
path that required one PN chip less time to traverse than the path
that was successfully demodulated by demodulator 400d and two PN
chip intervals less that the signal successfully demodulated by
demodulator 400b. The sample is PN descrambled in PN descrambling
element 404f. In the exemplary embodiment, PN descrambling element
404f descrambles the sample in accordance with two PN sequences
(PN.sub.I and PN.sub.Q) provided by PN generator 406.
[0125] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 412f and to pilot filter
410f. Pilot filter 410f uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, W.sub.pilot is the all zeroes Walsh sequence
for which the uncovering operation is a No Op. In this special
case, pilot filter 410f is simply a low pass filter which removes
the noise from the pilot signal. The complex conjugate of the
filtered pilot signal and the complex PN despread sequences are
multiplied in complex conjugate multiplier 412f which computes the
dot product of the pilot channel conjugate and the PN descrambled
sequence to provide a scalar sequence to Walsh sequence multiplier
414f.
[0126] Walsh sequence multiplier 414f multiplies the input scalar
sequence from complex conjugate multiplier 412f by a Walsh sequence
from Walsh generator 418. The multiplied sequence is then provided
to summing means 424.
[0127] After the demodulated signals from demodulators 400b, 400d
and 400f have been provided to summing element 424, summing element
424 sums the energies and outputs the summed energy values to
accumulator 426. Accumulator 426 performs the integration or
summation of the input symbols over the Walsh symbol interval.
[0128] Turning to FIG. 9, the baseband samples are provide to the
fat finger demodulator illustrated in FIG. 9 at three times the PN
chip rate. The baseband samples are provided to switch 502 which
switches each subsequent sample to a different line. Demodulators
500a and 500d demodulate the received signal that are delayed with
respect to one another by one PN chip interval. Similarly,
demodulators 500b and 500e demodulate a second set of samples
delayed with respect to one another by one PN chip interval.
Lastly, demodulator 500c demodulates a set of samples that is
unique from those provided to demodulators 500a, 500b, 500d and
500e. The theory behind the operation of the demodulator of FIG. 9
is identical to that of FIG. 8. Each demodulator demodulates the
signal that traversed a path that required a time to traverse
differing from one another by one half PN chip interval.
[0129] The first sample is provided through switch 502 onto line
499 to demodulator 500a. The sample is PN descrambled in PN
descrambling element 504a. In the exemplary embodiment, PN
descrambling element 504a descrambles the sample in accordance with
two PN sequences (PN.sub.I and PN.sub.Q) provided by PN generator
506.
[0130] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 512a and to pilot filter
510a. Complex conjugate multiplier 512a removes phase ambiguities
that are introduced by the propagation path. Pilot filter 510a
uncovers the pilot channel in accordance with the Walsh covering
for the pilot channel W.sub.pilot. In the exemplary embodiment,
W.sub.pilot is the all zeroes Walsh sequence for which the
uncovering operation is a No Op. In this special case, pilot filter
510a is simply a low pass filter which removes the noise from the
pilot signal. The complex conjugate of the filtered pilot signal
and the complex PN despread sequences are multiplied in complex
conjugate multiplier 512a which computes the dot product of the
pilot channel conjugate and the PN descrambled sequence to provide
a scalar sequence to Walsh sequence multiplier 514a.
[0131] Walsh sequence multiplier 514a multiplies the input scalar
sequence from complex conjugate multiplier 512a by the Walsh
traffic sequence from Walsh generator 518. The multiplied sequence
is then provided to combiner 524.
[0132] The first sample is redundantly provided through switch 502
on line 499 to delay element 522. Delay element 522 delays the
signal by one PN chip interval prior to providing the sample to
demodulator 500d. Thus, the signal successfully demodulated by
demodulator 500d will have traversed a propagation path that
required one PN chip less time to traverse than the path that was
successfully demodulated by demodulator 500a. The sample is PN
descrambled in PN descrambling element 504d. In the exemplary
embodiment, PN descrambling element 504d descrambles the sample in
accordance with two PN sequences (PN.sub.I and PN.sub.Q) provided
by PN generator 506.
[0133] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 512d and to pilot filter
510d. Pilot filter 510c uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, W.sub.pilot, is the all zeroes Walsh sequence
for which the uncovering operation is a No Op. In this special
case, pilot filter 510d is simply a low pass filter which removes
the noise from the pilot signal. The complex conjugate of the
filtered pilot signal and the complex PN despread sequences are
multiplied in complex conjugate multiplier 512d which computes the
dot product of the pilot channel conjugate and the PN descrambled
sequence to provide a scalar sequence to Walsh sequence multiplier
514d.
[0134] Walsh sequence multiplier 514d multiplies the input scalar
sequence from complex conjugate multiplier 512d by the Walsh
traffic sequence from Walsh generator 518. The multiplied sequence
is then provided to combiner 524.
[0135] After one third PN chip interval, switch 502 toggles so as
to put the next sample on input line 498 to demodulators 500b and
500e.
[0136] Within demodulator 500b, the sample is PN descrambled in PN
descrambling element 504b. In the exemplary embodiment, PN
descrambling element 504b descrambles the sample in accordance with
two PN sequences (PN.sub.I and PN.sub.Q) provided by PN generator
506. The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 512b and to pilot filter
510b. Pilot filter 510b uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, pilot filter 510b is simply a low pass filter
which removes the noise from the pilot signal. The complex
conjugate of the filtered pilot signal and the complex PN despread
sequences are multiplied in complex conjugate multiplier 512b which
computes the dot product of the pilot channel conjugate and the PN
descrambled sequence to provide a scalar sequence to Walsh sequence
multiplier 514b.
[0137] Walsh sequence multiplier 514b multiplies the input scalar
sequence from complex conjugate multiplier 512b by Walsh traffic
sequence from Walsh generator 518. The multiplied sequence is then
provided to combiner 524.
[0138] The second sample is redundantly provided through switch 502
on line 498 to delay element 520. Delay element 520 delays the
signal by one PN chip interval prior to providing it to demodulator
500e. Demodulator 500e successfully demodulates a signals that
traversed a path that took one PN chip less time to traverse than
the path successfully demodulated by demodulator 500b. The second
sample is PN descrambled in PN descrambling element 504e. In the
exemplary embodiment, PN descrambling element 504e descrambles the
sample in accordance with two PN sequences (PN.sub.I and PN.sub.Q)
provided by PN generator 506.
[0139] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 512e and to pilot filter
510e. Pilot filter 510e uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, pilot filter 510e is simply a low pass filter
that removes the noise from the pilot signal. The complex conjugate
of the filtered pilot signal and the complex PN despread sequences
are multiplied in complex conjugate multiplier 512e which computes
the dot product of the pilot channel conjugate and the PN
descrambled sequence to provide a scalar sequence to Walsh sequence
multiplier 514e.
[0140] Walsh sequence multiplier 514e multiplies the input scalar
sequence from complex conjugate multiplier 512e by the Walsh
traffic sequence from Walsh generator 518. The multiplied sequence
is then provided to combiner 524.
[0141] One third of a PN chip interval later, switch 502 switches
so as to provide the third base band sample onto output line 497,
which provides the sample directly to demodulator 500c. The sample
is PN descrambled in PN descrambling element 504c. In the exemplary
embodiment, PN descrambling element 504c descrambles the sample in
accordance with two PN sequences (PN.sub.I and PN.sub.Q) provided
by PN generator 506.
[0142] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 512c and to pilot filter
510c. Pilot filter 510c uncovers the pilot channel in accordance
with the Walsh covering for the pilot channel W.sub.pilot. In the
exemplary embodiment, W.sub.pilot is the all zeroes Walsh sequence
for which the uncovering operation is a No Op. In this special
case, pilot filter 510c is simply a low pass filter which removes
the noise from the pilot signal. The complex conjugate of the
filtered pilot signal and the complex PN despread sequences are
multiplied in complex conjugate multiplier 512c which computes the
dot product of the pilot channel conjugate and the PN descrambled
sequence to provide a scalar sequence to Walsh sequence multiplier
514c.
[0143] Walsh sequence multiplier 514c multiplies the input scalar
sequence from complex conjugate multiplier 512c by the Walsh
traffic sequence from Walsh generator 518 and provides the result
to combiner 524 combines the energies of the demodulated signals
from demodulators 500a, 500b, 500c, 500d and 500e and provides the
result to accumulator 526. As described previously there are many
alternative methods of combining the demodulated data all of which
are within the scope of the present invention. Accumulator 526
accumulates the combined energy values over the Walsh symbol
interval and outputs the result.
[0144] FIG. 10 illustrates a modification to FIG. 7 which is
applicable to all of the previous embodiments illustrated in FIGS.
6, 7, 8 and 9. FIG. 10 illustrates a modification to FIG. 7 which
allows for the elimination of switch 302. In FIG. 10, four
demodulators 600a, 600b, 600c and 600d are provided to demodulate
paths that are a fixed half PN chip distance from one another. The
received samples are provided at twice the PN chip rate to
demodulators 600a, 600b, 600c and 600d. Pilot filters 610a and 610c
ignore the even samples and pilot filters 610b and 610d ignore the
odd samples. Combiner 624 only combines demodulated odd samples
from demodulators 600a and 600c with demodulated even samples from
demodulators 600b and 600d.
[0145] The first sample is provided directly to demodulators 600a
and 600b and is delayed by one PN chip interval by delay elements
620 and 622 before being provided to demodulators 600c and 600d,
respectively. The sample is PN descrambled in PN descrambling
elements 604a, 604b, 604c and 604d. In the exemplary embodiment, PN
descrambling elements 604a, 604b, 604c and 604d descramble the
sample in accordance with two PN sequences (PN.sub.I and PN.sub.Q)
provided by PN generator 606. The complex descrambling operation is
performed as described above with respect to complex despreading
element 150a.
[0146] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 612a, 612b, 612c and 612d and
to pilot filter 610a, 610b, 610c and 610d. Complex conjugate
multiplier removes phase ambiguities that are introduced by the
propagation path. Pilot filters 610b and 610d ignore the first
sample and all future odd samples. Pilot filters 610a and 610c
uncover the pilot channel in accordance with the Walsh covering for
the pilot channel W.sub.pilot. In the exemplary embodiment,
W.sub.pilot is the all zeroes Walsh sequence for which the
uncovering operation is a No Op. In this special case, pilot filter
610a and 610c are simply low pass filters which remove the noise
from the pilot signal. The complex conjugate of the filtered pilot
signal and the complex PN despread sequences are multiplied in
complex conjugate multipliers 612a ahd 612c which compute the dot
product of the pilot channel conjugate and the PN descrambled
sequence to provide scalar sequences to Walsh sequence multipliers
614a and 614c.
[0147] Walsh sequence multipliers 614a and 614c multiply the input
scalar sequence from complex conjugate multipliers 612a and 612c by
the Walsh traffic sequence from Walsh generator 618. The multiplied
sequence is then provided to combiner element 624. Note that
although PN descrambling and Walsh multiplication are provided on
the first sample by demodulators 600b and 600d, combiner 624
ignores the data provided from demodulators 600b and 600d for odd
samples as erroneous.
[0148] The second sample is, then, provided directly to
demodulators 600a and 600b and is delayed by one PN chip interval
by delay elements 620 and 622 before being provided to demodulators
600c and 600d, respectively. The sample is PN descrambled in PN
descrambling elements 604a, 604b, 604c and 604d. In the exemplary
embodiment, PN descrambling elements 604a, 604b, 604c and 604d
descramble the sample in accordance with two PN sequences (PN.sub.I
and PN.sub.Q) provided by PN generator 606. The complex
descrambling operation is performed as described above with respect
to complex despreading element 150a.
[0149] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 612a, 612b, 612c and 612d and
to pilot filter 610a, 610b, 610c and 610d. Complex conjugate
multipliers remove phase ambiguities that are introduced by the
propagation path. Pilot filters 610a and 610c ignore the second
sample and all future even samples. Pilot filters 610b and 610d
uncover the pilot channel in accordance with the Walsh covering for
the pilot channel W.sub.pilot. In the exemplary embodiment,
W.sub.pilot is the all zeroes Walsh sequence for which the
uncovering operation is a No Op. In this special case, pilot filter
610b and 610d are simply low pass filters which remove the noise
from the pilot signal. The complex conjugate of the filtered pilot
signal and the complex PN despread sequences are multiplied in
complex conjugate multipliers 612b and 612d which compute the dot
product of the pilot channel conjugate and the PN descrambled
sequence to provide scalar sequences to Walsh sequence multipliers
614b and 614d.
[0150] Walsh sequence multipliers 614b and 614d multiply the input
scalar sequence from complex conjugate multiplier 612b and 612d by
the Walsh traffic sequence from Walsh generator 618. The multiplied
sequence is then provided to combiner element 624. Note that
although PN descrambling and Walsh multiplication are provided on
the second sample by demodulators 600a and 600c, combiner 624
ignores the data provided from demodulators 600b and 600d for odd
samples as erroneous.
[0151] After the demodulated signals from demodulators 600b and
600d have been provided to combiner element 624, combiner element
624 combines the energies and outputs the combined energy values to
accumulator 626. As described earlier, the combining operation can
take many forms all of which are within the scope of the present
invention. Accumulator 626 performs the integration or summation of
the input symbols over the Walsh symbol interval.
[0152] FIG. 11 illustrates a modification to FIG. 10 which is
applicable to all of the previous embodiments illustrated in FIGS.
6, 7, 8 and 9. FIG. 11 illustrates a modification to FIG. 10 which
allows for the elimination of all but one Walsh multiplier. In FIG.
10, four demodulators 650a, 650b, 650c and 650d are provided to
demodulate paths that are a fixed half PN chip distance from one
another. The received samples are provided at twice the PN chip
rate to demodulators 650a, 650b, 650c and 650d. Pilot filters 660a
and 660c ignore the even samples and pilot filters 660b and 660d
ignore the odd samples. Combiner 674 only combines demodulated odd
samples from demodulators 650a and 650c with demodulated even
samples from demodulators 650b and 650d.
[0153] The first sample is provided directly to demodulators 650a
and 650b and is delayed by one PN chip interval by delay elements
670 and 672 before being provided to demodulators 650c and 650d,
respectively. The sample is PN descrambled in PN descrambling
elements 654a, 654b, 654c and 654d. In the exemplary embodiment, PN
descrambling elements 654a, 654b, 654c and 654d descramble the
sample in accordance with two PN sequences (PN.sub.I and PN.sub.Q)
provided by PN generator 656.
[0154] The complex PN descrambled sequences are provided to a first
input of complex conjugate multipliers 662a, 662b, 662c and 662d
and to pilot filters 660a, 660b, 660c and 660d. Complex conjugate
multipliers remove phase ambiguities that are introduced by the
propagation path. Pilot filters 660b and 660d ignore the first
sample and all future odd samples. Pilot filters 660a and 660c
uncover the pilot channel in accordance with the Walsh covering for
the pilot channel W.sub.pilot. In the exemplary embodiment,
W.sub.pilot is the all zeroes Walsh sequence for which the
uncovering operation is a No Op. In this special case, pilot filter
660a and 660c are simply low pass filters which remove the noise
from the pilot signal.
[0155] The complex conjugate of the filtered pilot signal and the
complex PN despread sequences are multiplied in complex conjugate
multipliers 662a and 662c which compute the dot product of the
pilot channel conjugate and the PN descrambled sequence to provide
scalar sequences to combiner 674. Note that although PN
descrambling and Walsh multiplication are provided on the first
sample by demodulators 650b and 650d, combiner 674 ignores the data
provided from demodulators 650b and 650d for odd samples as
erroneous.
[0156] The second sample is, then, provided directly to
demodulators 650a and 650b and is delayed by one PN chip interval
by delay elements 620 and 622 before being provided to demodulators
650c and 650d, respectively. The sample is PN descrambled in PN
descrambling elements 654a, 654b, 654c and 654d. In the exemplary
embodiment, PN descrambling elements 654a, 654b, 654c and 654d
descramble the sample in accordance with two PN sequences (PN.sub.I
and PN.sub.Q) provided by PN generator 656. The complex
descrambling operation is performed as described above with respect
to complex despreading element 150a.
[0157] The complex PN descrambled sequences are provided to a first
input of complex conjugate multipliers 662a, 662b, 662c and 662d
and to pilot filters 660a, 660b, 660c and 660d. Complex conjugate
multipliers remove phase ambiguities that are introduced by the
propagation path. Pilot filters 660a and 660c ignore the second
sample and all future even samples.
[0158] Pilot filters 660b and 660d uncover the pilot channel in
accordance with the Walsh covering for the pilot channel
W.sub.pilot. In the exemplary embodiment, W.sub.pilot is the all
zeroes Walsh sequence for which the uncovering operation is a No
Op. In this special case, pilot filters 660b and 660d are simply
low pass filters which removes the noise from the pilot signal. The
complex conjugate of the filtered pilot signal and the complex PN
despread sequences are multiplied in complex conjugate multiplier
662b and 662d which compute the dot product of the pilot channel
conjugate and the PN descrambled sequence to provide scalar
sequences to combiner 674.
[0159] Combiner 674 combines the odd samples demodulated by
demodulators 650a and 650c with the even samples demodulated by
demodulators 650b and 650d. Combiner 674 can take many forms as
discussed previously with respect to the previous combiners. The
combined symbols are then provided to Walsh sequence multiplier
664.
[0160] Walsh sequence multiplier 664 multiplies the combined symbol
sequence by the Walsh traffic sequence W.sub.traffic which is
provided by Walsh sequence generator 668. The output from Walsh
sequence multiplier 664 is provided to accumulator 676. Accumulator
676 accumulates the Walsh multiplied sequence to provide Walsh
despread data.
[0161] FIG. 12 illustrates a modification to FIG. 7 which is
equally applicable to the modification of FIGS. 6, 7, 8 and 9. FIG.
12 provides for the demodulation of four paths that are one half PN
chip apart using two demodulators. Demodulator 700a demodulates two
paths that are one half PN chip apart and Demodulator 700b
demodulates two additional paths that are one half PN chip apart
from one another and one full PN chip from the paths demodulated by
demodulator 700a.
[0162] The samples are provided at twice the PN chip rate. The
samples are provided directly to demodulator 700a and are delayed
by one PN chip prior to being provided to demodulator 700b. The
sample is PN descrambled in PN descrambling elements 704a and 704b.
In the exemplary embodiment, PN descrambling elements 704a and 704b
descramble the sample in accordance with two PN sequences (PN.sub.I
and PN.sub.Q) provided by PN generator 706. The complex
descrambling operation is performed as described above with respect
to complex despreading element 150a.
[0163] The complex PN descrambled sequences are provided to a first
input of complex conjugate multiplier 712a and 712b and to pilot
filter 710a and 710b. Complex conjugate multipliers remove phase
ambiguities that are introduced by the propagation path.
[0164] In the first embodiment of pilot filters 710a and 710b,
pilot filters 710a and 710b consist of two independent filters. A
first of the two independent filters processes the odd samples and
a second of the two independent filters processes the even
samples.
[0165] In this embodiment, a first sample is provided to pilot
filter 710a and 710b, and is processed by the independent filter
within it which processes the odd samples. The output of the pilot
filters 710a and 710b are provided to complex conjugate multipliers
712a and 712b which multiply the conjugate of the pilot filter
output with the descrambled signal from PN descrambling element
704a and 704b. Note the odd descrambled symbols are complex
multiplied by the odd pilot symbols.
[0166] In this embodiment, a second sample is provided to pilot
filter 710a and 710b, and is processed by the independent filter
within it which processes the even samples. The output of the pilot
filters 710a and 710b are provided to complex conjugate multipliers
712a and 712b which multiply the conjugate of the pilot filter
output with the descrambled signal from PN descrambling element
704a and 704b. Note the even descrambled symbols are complex
multiplied by the even pilot symbols.
[0167] In a second embodiment of pilot filters 710a and 710b, each
pilot filter simply processes all of the samples.
[0168] The data from complex conjugate multipliers 712a and 712b
are provided to Walsh sequence multipliers 714a and 714b. The Walsh
traffic sequence is provided by Walsh generator 718 and the product
sequence is provided from Walsh sequence multipliers 714a and 714b
to combiner 724. Combiner 724 combines the Walsh sequence
multiplied data as described with respect to combiner 224. The
combined symbols are provided to accumulator 726 which accumulates
the energy over the Walsh symbol length.
[0169] FIG. 13 illustrates a modification to FIG. 12. FIG. 13
essentially performs the operation as described with respect to
FIG. 12 using the second implementation of the pilot filter.
Demodulator 800a demodulates two paths that are one half PN chip
apart and demodulator 800b demodulates two additional paths that
are one half PN chip apart from one another and one full PN chip
from the paths demodulated by demodulator 800a.
[0170] The samples are provided at twice the PN chip rate. The
samples are provided to sample combiners 834a and 834b. Sample
combiners 834a and 834b receive the samples at twice the PN chip
rate and sum together sample received one half PN chip apart and
provide the sum to demodulators 800a and 800b at the chip rate.
[0171] The first sample is provided to sample combiners 834 and is
provided through switches 832 to delay elements 828. Delay elements
828 delay the sample by one half PN chip interval before providing
the sample to a first summing input of summers 830. The second
sample is then provided to equalizers 834 and provided through
switches 832 to a second summing input of summer 830.
[0172] The two samples are summed together by summers 830 and the
output is provided by sample combiner 834a to demodulator 800a and
by sample combiners 834b to delay element 822. Delay element 822
delays the result from summer 830b by one PN chip interval before
providing it to demodulator 800b.
[0173] In demodulators 800a and 800b, the received summed samples
are provided to PN descrambling elements 804a and 804b. In the
exemplary embodiment, PN descrambling elements 804a and 804b
descramble the sample in accordance with two PN sequences (PN.sub.I
and PN.sub.Q) provided by PN generator 806. The complex
descrambling operation is performed as described above with respect
to complex despreading element 150a.
[0174] The complex PN descrambled sequences are provided to a first
input of complex conjugate multipliers 812a and 812b and to pilot
filters 810a and 810b. Complex conjugate multipliers remove phase
ambiguities that are introduced by the propagation path. Complex
conjugate multipliers 812a and 812b multiply the PN descramble
symbols by the conjugate of the pilot filter symbols. The data from
complex conjugate multipliers 812a and 812b are provided to Walsh
sequence multipliers 814a and 814b. The Walsh traffic sequence is
provided by Walsh generator 818 and the product sequence is
provided from Walsh sequence multipliers 814a and 814b to combiner
824. Combiner 824 combines the Walsh sequence multiplied data as
described with respect to combiner 224. The combined symbols are
provided to accumulator 826 which accumulates the energy over the
Walsh symbol length. Although the present invention is described
with respect to traditional PN sequences such as those in IS-95,
the present invention is equally applicable to other spreading
sequences such as Gold codes. Moreover, although coherent detection
using pilot channel offer significant benefits in system
performance, the present invention is equally applicable to non
coherent detection methods that do not use a pilot channel.
[0175] The previous description of the preferred embodiments is
provided to enable any person skilled in the art to make or use the
present invention. The various modifications to these embodiments
will be readily apparent to those skilled in the art, and the
generic principles defined herein may be applied to other
embodiments without the use of the inventive faculty. Thus, the
present invention is not intended to be limited to the embodiments
shown herein but is to be accorded the widest scope consistent with
the principles and novel features disclosed herein.
* * * * *